You are not properly constructing the extended SoapClient. return new SoapClient(...) should be parent::__construct(...). Constructor methods should never return anything.

The next issue, and this is the big one, the EarthTools web services (docs) do not talk SOAP. They're plain XML that you need to figure out how to a) construct the correct URLs for your needs (looks like you can do this already), and b) parse the response XML into PHP variables yourself.